1. Field of the Invention
This invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has positive temperature coefficient (PTC) of resistivity characteristics that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline polymer matrix is well known in the art, as disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is believed to be due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive particles linked together.
An organic PTC thermistor can be used as an overcurrent or overheat-protecting element, a self-regulating heater, and a temperature sensor. The characteristics which are required by these elements include a sufficiently low resistance value at room temperature in a quiescent state, a sufficiently high rate of change between the room-temperature resistance value and the resistance value in operation, and a minimal change of resistance upon repeated operation.
Electrically conductive particles used in organic PTC thermistors are typically carbonaceous particles such as carbon black and graphite. In order to reduce the resistance in a quiescent state of the thermistor, a large amount of carbonaceous particles must be dispersed in the matrix. This makes it difficult to increase the rate of resistance change, failing to provide satisfactory characteristics for protecting overcurrent or overheating.
This drawback can be overcome using metal particles having a lower resistivity than carbonaceous particles. For instance, the inventors proposed in JP-A 10-214705 and JP-A 11-168005 that the use of metal particles having spiky protuberances can find a compromise between a low room-temperature resistance and a high resistance change rate.
However, the inventors found that these organic PTC thermistors using metal particles lack reliability in that the room-temperature resistance increases during storage under severe conditions including a high temperature and a high humidity. Presumably the reasons why characteristics degrade during storage are that metal particles are oxidized on their surface to reduce their conductivity, that more metal particles agglomerate to break some conductive paths, and the like.